The Young and the Fast

Young animals are smaller than grown ones: that's one way we tell them apart. But the age at which juveniles achieve full size varies from species to species. And because size affects movement, young animals may move differently from their elders.

A five-year-old Gorgosaurus, a small tyrannosaur, probably weighed about 210 kilograms (460 pounds). Gorgosaurus reached full adult size--roughly 1000 kilograms (2,200 pounds)--at about age 18. Running is still possible at that weight, so the animal may have been a speedy adult.

In contrast, a 13-year-old T.rex equaled an adult Gorgosaurus in weight. And then the youngster really started to grow, adding 2.1 kilograms a day (nearly five pounds) for four years! Even an adolescent T. rex was unlikely to be a fast runner.

How Old Is That Dinosaur?

Sometimes animal bones, like the trunks of trees, accumulate yearly growth rings. The number of rings tells scientists the age of a specimen; the width reveals whether the animal was still growing. But only certain bones preserve a complete set of growth rings; unfortunately, the large limb bones that survive best as fossils are not among them.

T. rex bones are rare, so scientists knew little about the growth rates and ages of individual animals in this group...until now. Fossil finds over the past 10 years have tripled the number of fairly complete T. rex specimens. From them, scientists have identified bones with readable ring series. Now they can begin to reconstruct T. rex's life history.

Written in the Bones

To study the growth rings in bone, scientists examine a slice called a thin-section under a microscope. This thin section of a T. rex rib shows annual growth lines 12 through 19.

Size, Speed, and Safety

Attaining a weight of 1,000 kilograms (2,200 pounds) was an important milestone in the life of a young tyrannosaur. Below that weight, the animal could move fairly fast; above it, life shifted into the slow lane.

Fast running would have been useful to a juvenile dinosaur. The young are more vulnerable to predators than are their full-grown parents; speed might have helped to offset the risks of small size.

Elephants are capable of their fastest speeds at about age two. Still relatively small at that age, they are vulnerable to predators, so speed may provide a slight advantage.